In general, hydrogen gas obtained by steam reforming of hydrocarbon materials such natural gas, alcohols such as methanol, naphtha or the like is used as a fuel gas for fuel cells. In steam reforming reactions of these fuels, catalysts are also used, such as Ni, Cu, Zn or the like which are mainly used in a reducing atmosphere and carbon monoxide is generated in such reactions as a by-product in addition to hydrogen and carbon dioxide. When the thus formed carbon monoxide remains, it poisons a platinum group metal catalyst used in cell electrodes in fuel cells of a phosphoric acid type or polymer electrolyte type operating at low temperatures and sufficient power generation performance cannot be attained. For example, in polymer electrolyte type fuel cells operating at a particularly low temperature, the cell electrodes are poisoned in a short time and the power generation performance abruptly lowers even when a gas containing as low as 50 ppm of carbon monoxide is supplied.
For this reason, a carbon monoxide converting catalyst has been used to reduce the carbon monoxide concentration in the reformed fuel gas, followed by the use of a platinum group metal catalyst to eliminate the carbon monoxide by oxidation.
For example, Japanese Patent Laid-Open No. 5-201702 discloses a technique in which a platinum or rhodium catalyst prepared with alumina carriers is used to selectively oxidize and eliminate carbon monoxide at a low temperature. Japanese Patent Laid-Open No. 7-185303 discloses another technique in which catalyst temperature is maintained constant so as to more effectively eliminate carbon monoxide.
Even when these techniques are employed, however, the platinum group metal catalysts may be deteriorated in the activity depending upon operation conditions when used in a reducing atmosphere containing carbon monoxide in a high level. The poisoning by carbon monoxide may occur on the platinum group metal catalysts in the carbon monoxide removing apparatus which oxidizes and eliminates carbon monoxide in hydrogen gas obtained by reforming and converting reactions. The poisoning tends to occur particularly under the conditions where catalyst temperature is low and the space velocity of the catalyst is high. As a result, there arises a problem in that carbon monoxide cannot be eliminated sufficiently by the carbon monoxide removing apparatus.
The object of the present invention is to solve the above described problem.